Non-evaporable getter alloys

ABSTRACT

Non-evaporable getter alloys containing zirconium, vanadium, iron, manganese and one or more elements selected among yttrium, lanthanum and Rare Earths are described, having improved features of gas sorption, particularly of nitrogen, with respect to the known getter alloys.

The present invention relates to non-evaporable getter alloys.

Particularly, the invention relates to non-evaporable getter alloyswhich provide for a high efficiency in the sorption of gases,particularly of nitrogen.

Non-evaporable getter alloys, also known as NEG alloys, can sorbhydrogen in a reversible way and gases such as oxygen, water, carbonoxides and, in the case of some alloys, nitrogen, irreversibly.

A first use of these alloys is vacuum maintenance. Vacuum maintenance isrequested in the most various applications, for example in particlesaccelerators, in X-rays generator tubes, in flat displays of the fieldemission type and in thermally insulating evacuated interspaces, such asin thermal bottles (thermos), in Dewars or in the pipings for oilextraction and transportation.

The NEG alloys can also be used for removing the above mentioned gaseswhen they are present in traces inside other gases, generally noblegases. An example is the use in lamps, particularly the fluorescent oneswhich are filled with noble gases at pressures of a few tens ofmillibars, wherein the NEG alloy has the purpose of removing traces ofoxygen, water, hydrogen and other gases, thus maintaining the suitableatmosphere for the lamp functioning; another example of removal oftraces of the cited gases from other gases is the purification of inertgases, in particular for applications in microelectronic industry.

Generally these alloys have zirconium and/or-titanium as main componentsand comprise one or more elements selected among transition metals oraluminum.

NEG alloys are the subject matter of several patents. U.S. Pat. No.3,203,901 discloses Zr—Al alloys, and in particular the alloy havingweight percent composition Zr 84%-Al 16%, manufactured and sold by theapplicant under the name St 101; patent U.S. Pat. No. 4,071,335discloses Zr—Ni alloys, and in particular the alloy having weightcomposition Zr 75.7%-Ni 24.3%, manufactured and sold by the applicantunder the name St 199; U.S. Pat. No. 4,306,887 discloses Zr—Fe alloys,and particularly the alloy having weight percent composition Zr 76.6%-Fe23.4%, produced and sold by the applicant under the name St 198; U.S.Pat. No. 4,312,669 discloses Zr—V—Fe alloys, and in particular the alloyhaving weight composition Zr 70%-V 24.6%-Fe 5.4%, manufactured and soldby the applicant under the name St 707; U.S. Pat. No. 4,668,424discloses zirconium-nickel-mischmetal alloys, with optional addition ofone or more transition metals; U.S. Pat. No. 4,839,085 discloses Zr—V—Balloys, wherein E is an element selected among iron, nickel, manganeseand aluminum or a mixture thereof, U.S. Pat. No. 5,180,568 disclosesintermetallic compounds Zr₁M′₁M″₁, wherein M″ and M″, either alike ordifferent, are selected among Cr, Mn, Fe, Co and Ni, and in particularthe compound Zr₁Mn₁Fe₁ manufactured and sold by the applicant under thename St 909; U.S. Pat. No. 5,961,750 discloses Zr—Co—A alloys, wherein Ais an element selected among yttrium, lanthanum, Rare Earts or a mixturethereof, and particularly the alloy having weight composition Zr80.8%-Co 14.2%-A 5%, produced and sold by the applicant under the nameSt 787; finally, getter alloys based on Zr and V for use in gaspurifiers are described in various patent applications published in thename of the firm Japan Pionics, among which for example the applicationsKokai 5-4809, 6-135707 and 7-242401.

NEG alloys have different properties according to their composition. Forexample, the alloy St 101 is, among those mentioned, the best one aslong as hydrogen sorption is concerned, but requires, for working, anactivation treatment at relatively high temperatures, of at least 700°C.; the alloy St 198 has poor nitrogen sorption properties, therefore itis employed for the purification of this gas; the compounds described inU.S. Pat. No. 5,180,568 do not sorb hydrogen As a result of thesebehavior differences, the choice of the NEG alloy to be employed dependson the specific foreseen application. In particular, it may be statedthat, among these, the most largely used is the one named St 707, asdescribed in U.S. Pat. No. 4,312,669, thanks to its good sorptionqualities, in particular for hydrogen, and to the relatively lowactivation temperature required by this NEG alloy.

Removal of atmospheric gases is important in some applications. This isfor example the case of the thermal insulation, wherein the gases whichremain in the evacuated interspace during the manufacture have to beremoved: as a matter of fact, in order to maintain the production costswithin acceptable limits, the pumping of the interspace which is carriedout before the sealing thereof is generally interrupted after a fixedtime, generally leaving a residual pressure, although limited, in theinterspace itself. The sorption of the atmospheric gases is alsorequested in the currently studied application of the energy inertialaccumulators, better known with the definition “fly wheels”, which workon the principle of rotating an object of high mass at high speed in anevacuated chamber, vacuum is necessary in this application in order toprevent the rotating mass from losing energy because of the frictionwith the gases present in the chamber. In these applications,particularly important for the choice of the NEG alloys is the behaviortowards nitrogen, both because this gas forms about 80% of thecomposition of the atmosphere, and because it is the one, amongatmospheric gases (with the exception of the noble gases), which isremoved by the NEG with the highest difficulty.

The industrial application which currently requires the highestefficiency of undesired gases removal is purification of gases for thesemiconductor industry. As a matter of fact, it is known that impuritiesin the process gases can be incorporated into the layers which form thesolid state devices, thus causing electronic defects in them andtherefore production rejects. The degrees of purity which are presentlyrequested by the semiconductor industry are of the order of the ppt(10⁻¹² in atoms or molecules). Therefore, the availability of NEG alloyshaving very high efficiency of impurity sorption is necessary; as abovenoted, nitrogen is, among gases which represent the normal impurities ina process gas, the one which is removed with the highest difficulty fromthe NEG alloys.

Object of the present invention is therefore providing non-evaporablegetter alloys having high gas sorption efficiency, particularlynitrogen.

This object is obtained according to the present invention bynon-evaporable getter alloys comprising zirconium, vanadium, iron,manganese and at least one element selected among yttrium, lanthanum andRare Earths, having a percent composition of the elements variablewithin the following ranges (in the rest of the text, all percentagesand ratios are by weight, unless otherwise specified):

zirconium from 60 to 85%;

vanadium from 2 to 20%;

iron from 0.5 to 10%;

manganese from 2.5 to 30%; and

yttrium, lanthanum, Rare Earths or mixtures thereof from 1 to 6%.

The invention will be described in the following with reference to thedrawings, wherein:

FIGS. 1 to 5 show various different embodiments of getter devices usingthe alloys of the invention;

FIGS. 6 to 11 show the results of gas sorption tests under variousconditions by the alloys of the invention and a reference alloy.

The alloy according to the invention differ from the alloys known fromU.S. Pat. No. 4,312,669 because of the lower content of vanadium andiron, which are replaced by manganese and one element among yttrium,lanthanum and Rare Earths; from the alloys of U.S. Pat. No. 4,668,424because these do not involve the use of vanadium and of manganese, andrequire instead the presence of nickel in quantities between 20 and 45%by weight; from the alloys of U.S. Pat. No. 4,839,085 because these donot require the use of yttrium, lanthanum or Rare Earths and containgenerally, with respect to the alloys of the invention, higherquantities of vanadium and lower quantities of iron and manganese; fromthe compounds of U.S. Pat. No. 5,180,568, because these are ternaryintermetallic compounds Zr₁M′₁M″₁ which do not contain vanadium oryttrium, lanthanum and Rare Earths; and from the alloys of U.S. Pat. No.5,961,750 which require the presence of cobalt and do not require thepresence of vanadium iron and manganese. As above mentioned and widelydescribed in the following, these differences in the composition resultin notable differences in the gas sorption, particularly as far asnitrogen is concerned.

With zirconium contents lower than 60%, the performances of gas sorptionof the alloys of the invention decrease, whereas contents of thiselement higher than 85% cause the alloys to be too plastic and difficultto work in the production of getter devices. The contents of othercomponents of the alloys which are outside the indicated percentagesgenerally involve reductions of the gas sorption features, particularlyof nitrogen in the case of high vanadium contents and of hydrogen forhigh iron or manganese contents. Further, it has been found that alloyscontaining vanadium less than 2% are too pyrophoric and thereforedangerous to be produced and handled. Finally, percentages higher than6% of yttrium, lanthanum, Rare Earths or mixtures thereof do not improvethe sorption features of the alloys, but cause them to be unstable inthe air with resulting problems of storage before use. Particularlyconvenient for the invention is the use, instead of the last mentionedelements, of mischmetal (also indicated simply MM in the following).Various commercial mixtures are identified with this name, comprisingabove all cerium, lanthanum and neodymium, and minor quantities of otherRare Earths, of lower costs with respect to the pure elements. The exactcomposition of the mischmetal is not important, because the abovementioned elements have similar reactivities, so that the chemicalbehavior of the different available types of mischmetals is essentiallyconstant even if the content of the single elements is varied, so thatthe exact composition of this component does not have an influence overthe working features of the alloys according to the invention.

Within the indicated ranges, are preferred the alloys having a contentof:

zirconium varying between about 65 and 75%, and, even more preferablybetween about 67 and 70%;

vanadium 2.5 to 15%;

manganese 5 to 25%;

iron/vanadium ratio comprised between 1:4 and 1:5.

Particularly preferred among the alloys of the invention are an alloyhaving composition Zr 70%-V 15%-Fe 3.3%-Mn 8.7%-MM 3% and an alloy ofcomposition Zr 69%-V 2,6%-Fe 0.6%-Mn 24.8%-MM 3%.

The alloys of the invention can be prepared by fusion in an ovenstarting from pieces or powders of the component metals, taken inproportions corresponding to the final desired composition. Thetechniques of fusion in an arc oven under an inert gas atmosphere, forinstance under a pressure of 300 mbars of argon; or in an inductionoven, under vacuum or an inert gas are preferred. It is anyway possiblethe use of other techniques for the preparation of alloys which areusual of the metallurgical industry.

In practical applications, the alloys of the invention are used in theform of pellets of the getter material alone or on a support or inside acontainer. In any case, the use of alloys in the form of powders havingparticle size generally lower than 250 μm and preferably between 125 and40 μm is preferred. With larger particle size an excessive reduction ofthe specific surface of the material (surface area per weight unit)takes place, whereas particle size values lower than 40 μm, although canbe used and requested for some applications, cause some problems in theproduction steps of the getter devices (thin powders are more difficultto be moved by automatized means and are more pyrophoric with respect topowders having larger particle size).

The NEG alloys of the invention can be activated at temperaturescomprised between 300 and 500° C. for periods between 10 minutes and 2hours. The effect of the temperature prevails over the treatment time,and an activation at 400° C. for 10 minutes allows to obtain a nearlycomplete activation

Once activated, these alloys are able to work for the sorption of gasessuch as hydrogen, carbon oxide, and above all nitrogen, already at theroom temperature, with properties similar to the known alloys forhydrogen and better ones for carbon oxide and nitrogen. Generally themaximum temperature of use is about 500° C., not to compromise thestability and functionality of the device wherein they are inserted. Theoptimal working temperatures of these alloys depend on the specificapplications; for instance, in the case of the interspaces for thermalinsulation the temperature is determined by that of the warmest wall ofthe insterspace itself, in the case of the “fly wheels” the temperatureis the room temperature and in the purification of gases the temperatureis generally between about 300 and 400° C.

In the case of hydrogen, as for all known NEG materials, the sorption isreversible so that the sorption features are evaluated in terms ofequilibrium hydrogen pressure on the alloy as a function of thetemperature and of the quantity of sorbed hydrogen. From this point ofview the sorption of hydrogen by the alloys of the invention is verygood, and similar to that of the mentioned alloy St 707, that is themost widely used getter alloy. The alloy of the invention also have atroom temperature, with respect to the alloy St 707 in the sameconditions, sorption capacity up to 15 times greater for nitrogen and upto 10 times greater for Co.

As already mentioned, the forms of the getter devices which can beprepared by using the alloys of the invention are the most various,comprising for example pellets formed only of powders of the getteralloy, or of these on a support, generally metallic. In both cases theconsolidation of the powders can be carried out by compression or bycompression followed by sintering. The pellets made only of compressedpowders find an application for example in thermal insulation and in gaspurifications. In the cases wherein the powders are supported, steel,nickel or nickel alloys can be used as support material. The support canbe simply in the form of a band on the surface of which the powders ofthe alloy are adhered by cold rolling or by sintering after depositionby various techniques. Getter devices obtained from similar bands can beused in lamps. The support can also be formed of a proper containerhaving various shapes, wherein the powders are inserted generally bycompression, or even without compression in some devices wherein thecontainer is provided with a porous septum, permeable to the passage ofgases but able to retain powders; the latter configuration isparticularly suitable for the application of the “fly wheels”, whereinpowder of a moisture sorber material, such as calcium oxide, can beadditioned to the getter alloy. Some of these possibilities arerepresented in FIGS. 1 to 5, wherein FIG. 1 shows a pellet 10 made onlyof compressed powders of NEG alloy according to the invention. FIG. 2represents a NEG device 20, having a shape particularly suitable for theuse in lamps, obtained by cutting along parallel lines, orthogonal tothe longitudinal direction, a band 21 formed of a metal support 22 onwhich powders 23 of an alloy of the invention are present; the nextdevice of the type 20 is obtained by cutting the band along dotted lineA-A′. FIG. 3 shows in section a device 30 formed of an upperly openmetal container 31, wherein powders 32 of NEG alloy are provided. FIG. 4shows in section a device 40 formed of a metal container 41 whereinpowders 42 of a NEG alloy are provided, having an upper opening closedby a porous septum 43. Finally, FIG. 5 shows a device 50 similar to thatof the preceding drawing and particularly suitable in the application“fly wheels”, wherein powders of a NEG alloy 51 of the invention andpowders of a moisture sorbing material 52 are provided.

The invention will be now further illustrated by means of the followingexamples. These non limiting examples show some embodiments which areintended to teach those skilled in the art how to put the invention intopractice and to represent the best considered way for carrying out theinvention.

EXAMPLE 1

This example relates to the preparation of an alloy of the invention.100 g of an alloy having the composition Zr 70%-V 15%-Fe 3.3%-Mn 8.7%-MM3% are produced by melting in an induction oven, in proportionscorresponding to the desired composition, Zr, Mn, MM and a commercialV—Fe alloy containing about 81.5% by weight of vanadium. The mischmetalused has the weight percent composition of 50% cerium, 30% lanthanum,15% neodymium, and the remaining 5% of other Rare Earths. The alloyingot is ground under an argon atmosphere, in a pall mill and the powderis sieved, thus recovering the fraction having particle size of 40-128μm.

EXAMPLE 2

This example relates to the preparation of a second alloy of theinvention. The test of example 1 is repeated, but starting fromdifferent quantities of Zr, Mn, MM and V—Fe alloy, so as to obtain analloy having composition Zr 69%-V 2.6%-Fe 0.6%-Mn 24.8%-MM 3%.

EXAMPLE 3 (COMPARATIVE)

This example relates to the preparation of an alloy according to theknown art, to be used for example in the following examples; this alloyis taken as a reference because it is the NEG material which is mostcommonly used in application such as thermal insulation and gaspurification. 100 g of St 707 alloy are produced, by operating asdescribed in example 1, by using Zr and V—Fe alloy in proportionscorresponding to the desired composition.

EXAMPLE 4

This example refers to a measure of the nitrogen sorption properties byan alloy of the invention 0.2 g of powder prepared in example 1 areactivated at 500° C. for 10 minutes, and are then introduced in ameasure chamber. The nitrogen sorption test is carried out by followingthe procedure described in standard ASTM F 798-82, by operating at theroom temperature and with a nitrogen pressure of 4×10⁻⁶ mbars. The testresults are reported in a graphic as curve 1 in FIG. 6, as sorptionvelocity (indicated with S and measured in cm³ of gas sorbed per second,normalized per gram of alloy) as a function of the quantity of sorbedgas (indicated with Q and measured in cm³ of gas multiplied by thepressure of measure in mbars and normalized per gram of alloy).

EXAMPLE 5

The test of example 4 is repeated, by using 0.2 g of powder of example2. The results of the test are reported in a graph as curve 2 in FIG. 6.

EXAMPLE 6 (COMPARATIVE)

The test of example 4 is repeated by using 0.2 g of powder of example 3.The test results are reported in a graph as curve 3 in FIG. 6.

EXAMPLE 7

The test of example 4 is repeated, but using CO as the test gas. CO isused as the test gas because it is one of the gases which are found mostcommonly in evacuated spaces, such as the interspaces for thermalinsulation The test results are reported in a graph as curve 4 in FIG.7.

EXAMPLE 8

The test of example 7 is repeated by using 0.2 g of powder of example 2.The test results are reported in a graph as curve 5 in FIG. 7.

EXAMPLE 9 (COMPARATIVE)

The test of example 7 is repeated, by using 0.2 g of powder of example3. The test results are reported in a graph as curve 6 in FIG. 7.

EXAMPLE 10

The test of example 4 is repeated, but using hydrogen as the test gas.Hydrogen, together with CO, is one of the gases present in greatestquantity in evacuated spaces. The test results are reported in a graphas curve 7 in FIG. 8.

EXAMPLE 11

The test of example 10 is repeated, by using 0.2 g of powder of example2. The test results are reported in a graph as curve 8 in FIG. 8.

EXAMPLE 12 (COMPARATIVE)

The test of example 10 is repeated, by using 0.2 g of powder of example3. The test results are reported in a graph as curve 9 in FIG. 8.

EXAMPLE 13

The test of example 4 is repeated, but maintaining in this case thesample at 300° C. during the test. The test results are reported in agraph as curve 10 in FIG. 9.

EXAMPLE 14 (COMPARATIVE)

The test of example 13 is repeated, by using 0.2 g of powder of example3. The test results are reported in a graph as curve 11 in FIG. 9.

EXAMPLE 15

The test of example 4 is repeated, by using in this case, instead ofloose powders, a 2 mm high pellet, having 4 mm of diameter and about 125mg of weight, produced with the powder prepared as described inexample 1. The results of the test are reported in a graph as curve 12in FIG. 10.

EXAMPLE 16 (COMPARATIVE)

The test of example 15 is repeated, by using a pellet of powderaccording to example 3, having the same size as the pellet of example15. The results of the test are reported in a graph as curve 13 in FIG.10.

EXAMPLE 17

The test of example 15 is repeated, by using this time CO as the testgas. The results of the test are reported in a graph as the curve 14 inFIG. 11.

EXAMPLE 18 (COMPARATIVE)

The test of example 17 is repeated, by using a pellet of powder ofexample 3 having the same size of the pellet of example 17. The resultsof the test are reported in a graph as curve 15 in FIG. 11.

A particularly important factor for evaluating a NEG alloy for practicalapplications, above all when working at room temperature is foreseen, isthe sorption capacity at a certain sorption velocity. In fact, in thenormal applications the theoretical sorption capacity of the NEG alloys,which is determined as the stoichiometric completion of the reactionbetween the metal components and the sorbed gases, is never reached, andgenerally the lower is the working temperature, the smaller is thedegree of progress of said reaction. Therefore, from the practical pointof view, it is assumed as the capacity of a getter alloy the one atwhich its sorption velocity has decreased, from the initial value, to aminimum value acceptable for the application; further, it is assumedthat this minimum value is equal to the velocity with which the gasespenetrate inside the evacuated space, because of release or permeationfrom the walls; in the case of applications in purification, saidminimum value must be at least equal to the flow of the impurities whichcome onto the alloy. These practical conditions ensure that the getteralloy is able to absorb completely the quantity of gaseous impuritieswith which it is in contact. By analyzing the results of the tests itcan be noticed that the alloys of the invention have gas sorptionproperties better than the alloy St 707; particularly, the capacity fornitrogen at room temperature is about 5-15 times greater than the alloySt 707 in the case of loose powders (FIG. 6), and about 3-5 timesgreater in the case of pellets (FIG. 10); the capacity for CO at theroom temperature is about 3-5 times greater than for the St 707 alloy inthe case of loose powders (FIG. 7) and about 6-10 times greater in thecase of pellets (FIG. 11); the capacity for hydrogen of the powderalloys of the invention is better than that of the alloy St 707 at theroom temperature (FIG. 8); finally, even at 300° C. powders of an alloyof the invention show nitrogen capacities higher than powders of thealloy St 707 (FIG. 9).

What is claimed is:
 1. Non-evaporable getter alloys having high gassorption efficiency, particularly for nitrogen, comprising zirconium,vanadium, iron, manganese and at least one element selected amongyttrium, lanthanum and Rare Earths, having a weight percent compositionof the elements which can vary within the following ranges: zirconiumfrom 60 to 85%; vanadium from 2 to 20% iron from 0.5 to 10%; manganesefrom 2.5 to 30%; and yttrium, lanthanum, Rare Earths or mixtures thereoffrom 1 to 6%.
 2. Alloys according to claim 1, wherein the weightpercentage of zirconium is comprised between about 65 to 75%.
 3. Alloysaccording to claim 2, wherein the weight percentage of zirconium iscomprised between about 67 and 70%.
 4. Alloys according to claim 1,wherein the weight percentage of vanadium is comprised between about2.5% and 15%.
 5. Alloys according to claim 1, wherein the weightpercentage of manganese is comprised between about 5 and 25%.
 6. Alloysaccording to claim 1, wherein the weight ratio between iron and vanadiumis between 1:4 and 1:5.
 7. An alloy according to claim 1 having thecomposition Zr 70%-V 15%-Fe 3.3%-Mn 8.7%-mischmetal 3%.
 8. An alloyaccording to claim 1 having the composition Zr 69%-V 2.6%-Fe 0.6%-Mn24.8%-mischmetal 3%.
 9. Getter devices comprising the alloys of claim 1in powder form.
 10. Devices according to claim 9, wherein said alloyshave particle size lower than 250 μm.
 11. Devices according to claim 9,wherein the powders have particle size comprised between 125 and 40 μm.12. Devices (10) according to claim 9, formed of pellets made only ofpowders of the getter alloy.
 13. Devices (20) according to claim 9,obtained cutting along parallel lines in the longitudinal direction aband (21) formed of a metal support (22) on which powders (23) of agetter alloy are provided.
 14. Devices (30) according to claim 9, formedof powders of a getter alloy (32) inside an upperly open metal container(31).
 15. Devices (40) according to claim 9, formed of powders of agetter alloy (42) inside a metal container (41) having an upper openingclosed by a porous septum (43).
 16. Devices (50) according to claim 15containing, besides the powders of the getter alloy (51), powders of amoisture sorbing material (52).
 17. Thermally insulating manufacturedarticles comprising an evacuated interspace, wherein the interspacecomprises an alloy according to claim
 1. 18. Manufactures articlesaccording to claim 17, wherein the interspace contains getter devicesaccording to claim
 11. 19. Gas purifiers containing an alloy accordingto claim
 1. 20. Purifiers according to claim 19, containing getterdevices according to claim
 12. 21. Lamps containing an alloy accordingto claim
 1. 22. Lamps according to claim 21, containing getter devicesaccording to claim
 13. 23. Evacuated chambers of inertial energyaccumulators containing an alloy according to claim
 1. 24. Chambersaccording to claim 23 containing devices according to claim
 15. 25.Chambers according to claim 23 containing devices according to claim 16.